Chromium plating is an electrochemical process that involves the electrodeposition of chromium onto a substrate from a chromium electrolyte. Two common types of chromium plating are hard chromium plating and decorative chromium plating. Hard chromium plating involves the application of a heavy coating of chromium onto steel substrates, typically to prevent wear, and exists in thicknesses in the range of about 10 to about 1000 μm. Decorative chromium plating applies a much thinner layer of chromium, in the range of about 0.25 to about 1.0 μm, and provides an extremely thin but hard coating for aesthetic purposes to achieve a shiny, reflective surface and/or protect against tarnish, corrosion and scratching of the metal beneath.
For decorative purposes, the chromium is generally applied over a coating of nickel. The chromium provides a hard, wear-resistant layer and excellent corrosion performance due to the chromium layer being cathodic with respect to the underlying nickel deposit. Thus, the underlying nickel layer becomes the anode in the corrosion cell and corrodes preferentially, leaving the chromium layer uncorroded.
Decorative chromium has traditionally been electroplated from electrolytes containing hexavalent chromium using, for example, an aqueous chromic acid bath prepared from chromic oxide (CrO3) and sulfuric acid. However, electrolytes based on trivalent chromium ions have also been developed. The incentive to use electrolytes based on trivalent chromium salts arises because hexavalent chromium presents serious health and environmental hazards. In addition, hexavalent chromium ions and solutions from which they can be plated have technical limitations including the high cost of disposing of plating baths and rinse water. Furthermore, the operation of plating from baths containing substantially hexavalent chromium ion has operational limits which increase the probability of producing commercially unacceptable deposits.
Chromium deposits obtained from electrolytes based on hexavalent chromium are essentially pure chromium and have a uniform and invariant color. A thin oxide layer forms on the top of the coatings, providing a blue/white appearance. However, there is a market demand for coatings having a darker hue and it is desirable to develop electrolytes that are capable of producing coatings having a darker hue. A partial solution to this problem can be obtained by electrodepositing the chromium coatings from electrolytes based on trivalent chromium. However, the electrolytes based on trivalent chromium still produce coatings that are not dark enough to fill the needs of certain customers and a demand continues to exist for improved means of producing darker chromium based coatings.